Many medical procedures are performed using minimally invasive surgical techniques wherein one or more devices are inserted through one or more small incisions into a patient's body. For example, a cardiac arrhythmia may be treated through selective ablation of cardiac tissue to eliminate the source of the arrhythmia. Radio frequency energy, microwave energy, laser energy, extreme heat, or extreme cold may be provided by the ablation device to ablate the tissue.
One category of such ablation devices is the minimally-invasive, catheter-based device that is introduced into the vasculature and used to treat tissue by cooling contact. Such catheter-based devices, henceforth generically referred to herein simply as “catheters” have an elongated body through which a cooling fluid circulates to a tip portion which is adapted to contact and cool tissue. The cooling fluid used with such catheters may be a low temperature fluid, or cryogens. In general, the catheters may be used to lower the temperature of tissue, such as cardiac wall tissue, to an extent such that signal generation or conduction ceases and allows one to map or confirm that the catheter is positioned at a particular lesion or arrhythmia conduction site. More recently, cryoablation catheters have been configured for ablation treatment, to cool the tissue to a much lower level at which freezing destroys the viability of the tissue, and, in the case of cardiac tissue, permanently removes it as a signal generating or signal conducting locus. Such devices are also useful for tissue destruction in other contexts, such as the ablation of tumorous, diseased, precancerous or congenitally abnormal tissue.
The catheters may be adapted for endovascular insertion, or for insertion along relatively confined pathways, for example through a body lumen, or through a small incision to and around intervening organs, to reach an intended ablation site. As such, they are characterized by a relatively elongated body through which the cooling fluid circulates, and a tip or distal end portion where the cooling is to be applied. The requirement that the coolant be localized in its activity poses stringent constraints on a working device. For example, when the catheter contact must chill tissue to below freezing, the coolant itself must attain a substantially lower temperature. The rate of cooling is limited by the ability to supply coolant and circulate it through the active contact region, and the efficacy of the contact region itself is further limited by geometry and physical properties that affect its ability to conduct heat into the tissue.
Furthermore, it is generally desirable to control the direction of the cryogenic fluid flow to only the target tissue sites. In some procedures, a spot tissue ablation procedure—where the fluid flow is directed at a specific site—may be acceptable. For other procedures, it may be more therapeutically effective if the fluid flow is directed along a predetermined line, or a single elongate ring or linear lesion created in a single ablative step. However, the small dimensions of the catheter assembly have the result that flow conditions existing within the catheter tip are turbulent and chaotic. This arises in part because the high pressure release of fluid in a relatively small chamber at the tip of the catheter and/or its recirculation back via a return conduit from the tip region involve relatively turbulent fluid flow conditions, so that the precise control of directional contact on the tissue may be subject to rather wide variations. Thus, while conventional catheter arrangements have been found to provide a high level of cooling, the introduction at high pressure into the small expansion chamber results in cavitation, turbulence and irregular fluid flow evolving in the short distance and brief time between the jet spray of expanding coolant and the lower pressure conditions existing at the proximal end of the chamber adjacent the coolant return passage.
Accordingly, cryoablation catheters could benefit from improved techniques and devices for providing uniform and evenly controlled flow of the thermal transfer fluid onto the targeted tissue cells.